The present invention pertains generally to coupling assemblies for thermally connecting a cryocooler with an apparatus that is to be cooled. More particularly, the present invention pertains to a method for cooling a superconducting device by using a sleeve assembly which thermally interconnects two stages of a cryocooler with two different components of a superconducting device simultaneously. The present invention particularly, though not exclusively, pertains to a method for using a sleeve assembly to thermally disconnect the pulse tube, two stage cryocooler from a superconducting device without compromising the thermal condition of the superconducting device.
It is well known that superconductivity is accomplished at extremely low temperatures. Even the so-called high temperature superconductors require temperatures which are as low as approximately twenty degrees Kelvin. Other not-so-high temperature superconductors require temperatures which are as low as approximately four degrees Kelvin.
In any case, there are numerous specialized applications for using superconducting devices that require low temperatures. One specialized application, for example, involves medical diagnostic procedures using magnetic resonance imaging (MRI) techniques. When used for medical diagnosis, MRI techniques require the production of a very strong and substantially uniform magnetic field. If superconducting magnets are used to generate this strong magnetic field, some type of refrigeration apparatus will be required to attain the low operational temperatures that are necessary.
To attain the low operational temperatures that are necessary for a superconducting device, the refrigeration apparatus typically includes separate cryogenic units or cryocoolers that are thermally connected with the superconducting device. During operation of the superconducting device, such a connection is essential. There are times, however, when it is desirable for the cryocooler to be selectively disconnected or disengaged from the superconducting device. For example, during repair or routine maintenance of the cryocooler in a refrigeration apparatus, it is much easier to work on the cryocooler when it is disconnected from the superconducting device it has been cooling. Importantly, when so disengaged, the cryocooler can be warmed to room temperature for servicing. Any disengagement of the cryocooler from the superconducting device, however, must allow for a reengagement. Further, it is desirable that the superconducting device be held at a very low temperature during disengagement.
As it is known to persons skilled in the pertinent art, new generation cryocoolers, such as xe2x80x9cPulse Tubesxe2x80x9d, cannot be xe2x80x9cguttedxe2x80x9d out and rebuilt as can the older generation cryocoolers. Instead, these pulse tube cryocoolers must either be entirely replaced or warmed to room temperature for servicing. It is, therefore, necessary for these new generation cryocoolers to use a refrigeration apparatus or a sleeve to cool a superconducting device. Because the entire pulse tube needs to be removed for servicing, the pulse tube cryocoolers cannot be directly and permanently bolted to the sleeve and, thus, the superconducting device. Further, the pulse tube internals cannot be removed independently as they can in many Gifford McMahon (GM) two stage cryocoolers.
For an effective thermal connection, it is known that the efficacy of heat transfer from one body to another body is dependent on several factors. More specifically, the amount of heat (Q) that is conductively transferred through a solid body or conductively transferred from one body to another body through a gas or liquid can be mathematically expressed as:
xe2x80x83Q=k(A/L)xcex94T
In the above expression, k is the coefficient of thermal conductivity; A is the solid bodies cross-sectional area, or the surface area in contact between the two bodies for gas or liquid conduction; L is the solid bodies thermal length or the gap distance between the bodies; and xcex94T is the temperature differential across the solid or between the two bodies. From this expression, it can be appreciated that in order to effectively cool one body (e.g. a superconducting device) with another body (e.g. a cryocooler) the transfer of heat, Q, must be accomplished. When the temperature differential between the bodies is desired to be very low, and for a given coefficient of thermal conductivity, it is necessary that the ratio of A/L be sufficiently high.
For any two separate bodies that are in contact with each other, even though they may be forced together under very high pressures, there will always be some average gap distance, L, between the interfacing cross-sectional surface areas of the bodies. For the case wherein there is a vacuum in the gaps, the gaps can create undesirable thermal insulators. Accordingly, it may be beneficial to have these gaps filled with a gas, such as helium. If this is done, heat transfer between the bodies in contact can result from a) solid conduction where there is actual contact between the bodies; b) molecular/gas conduction across the helium-filled gaps; and possibly c) liquid conduction in gaps where the gas has liquefied.
In light of the above, it is an object of the present invention to provide a method for cooling two components of a superconducting device by using a sleeve assembly that thermally interconnects two stages of a pulse tube cryocooler with the superconducting device. Another object of the present invention is to provide a method for cooling a superconducting device by using a sleeve assembly which allows the pulse tube, two stage cryocooler to be thermally disengaged from the superconducting device while the very low temperature of the superconducting device is substantially maintained. Still another object of the present invention is to provide a method for cooling a superconducting device which is effectively easy to implement and comparatively cost effective.
The present invention is directed to a method for cooling a superconducting device by using a sleeve assembly which thermally interconnects a pulse tube, two stage cryocooler with a superconducting device. For the present invention, the sleeve assembly has a heat transfer cylinder, a heat transfer receptacle and a midsection which interconnects the heat transfer cylinder with the heat transfer receptacle.
In more detail, the midsection of the sleeve assembly is hollow and elongated and defines a passageway between the heat transfer cylinder and the heat transfer receptacle. The heat transfer cylinder of the present invention is also hollow and is annular-shaped, having an inner surface and an outer surface. The heat transfer receptacle is formed with a recess and has an inner surface and an outer surface. Importantly, the inner surface of the heat transfer receptacle that defines the recess is tapered. Both the heat transfer cylinder and heat transfer receptacle are preferably made of copper, aluminum or any other high thermal conductivity material. Furthermore, the midsection of the sleeve assembly is preferably made of stainless steel or any other low thermal conductivity material known in the art.
The structure of the sleeve assembly is dimensioned for the engagement with a cryocooler which includes a cooling element and a tapered cooling probe. As contemplated for the present invention, the cryocooler is moveable relative to the sleeve assembly between a first configuration wherein the cryocooler is engaged with the sleeve assembly, and a second configuration wherein the cryocooler is disengaged from the sleeve assembly. Specifically, the two stages of the cryocooler will thermally engage and disengage with the two components of the superconducting device simultaneously through the sleeve assembly.
In operation, the sleeve assembly is engaged with the cryocooler when the cryocooler is juxtaposed with the sleeve assembly to establish thermal communication between the cryocooler and the superconducting device through the sleeve assembly. In more detail, when juxtaposed, the tapered cooling probe of the cryocooler is urged against the heat transfer receptacle of the sleeve assembly to establish thermal communication therebetween. As stated above, the inner surface of the heat transfer receptacle is tapered for mating engagement with the tapered cooling probe of the cryocooler. This engagement, however, will not be perfect. Always, there is an average gap distance between the inner surface of the heat transfer receptacle and the tapered cooling probe of the cryocooler. As contemplated for the present invention, this gap distance varies within the range between zero and approximately two thousandths of an inch (0-0.002 inches). Importantly, under these conditions, the gap ratio, A/L, in the above expression for Q will be in the range between approximately 10,000 in2/in to approximately 50,000 in2/in. Consequently, there can be effective heat flow, Q, even though the temperature differential, xcex94T, between the heat transfer receptacle and the tapered cooling probe is small.
When the cryocooler is engaged with the sleeve assembly (first configuration), the cooling element of the cryocooler is positioned at a very small gap distance from the inner surface of the heat transfer cylinder. Importantly, this gap distance needs to be small enough to establish effective thermal communication between the cooling element and the heat transfer cylinder. For the present invention, this gap distance will vary within the range between approximately one thousandth of an inch to approximately five thousandths of an inch (0.001-0.005 inches). Although the gap ratio, A/L, in this case will be higher than it is for the receptacle/probe interface, there will still be effective heat flow, Q.
In order for the cryocooler and sleeve assembly to move between the first (engaged) and second (disengaged) configurations, an expandable bellows is provided which joins the heat transfer cylinder of the sleeve assembly with the room temperature section of the cryocooler and creates an enclosed chamber therebetween. In operation, the bellows allows the cryocooler to be separated from the sleeve assembly with a space therebetween which will maintain a gaseous thermal insulation between the cryocooler and the sleeve assembly. Stated another way, there will be sufficient thermal insulation between the sleeve assembly and the cryocooler to maintain the sleeve assembly at a substantially same low temperature when the cryocooler is disengaged from the sleeve assembly and is warmed to room temperature.
It is important for the sleeve assembly to maintain two substantially low temperatures for it to continually cool the two separate components of the superconducting device. To do this, the sleeve assembly of the present invention is operationally connected to the superconducting device by a proximal conductor and a distal conductor. In more detail, the proximal conductor is attached between the outer surface of the heat transfer cylinder and a thermal shield of the superconducting device to establish thermal communication therebetween. Further, the distal conductor is attached between the outer surface of the heat transfer receptacle and the superconducting wires of the superconducting device to establish thermal communication therebetween.
By way of a pipe, helium gas is pumped selectively into and from the chamber of the sleeve assembly. As contemplated for the present invention, the introduction of helium gas into the space between the cryocooler and the sleeve assembly will prevent a vacuum from forming when the cryocooler is disengaged and displaced from the sleeve assembly. Also, helium gas is useful to establish molecular conduction between the sleeve assembly and the cryocooler for an effective thermal connection therebetween when these two components are engaged with each other.